1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) and, more particularly, to control of a transmission that switches from drive-to-reverse or reverse-to-drive preparatory to an engine restart event.
2. Description of the Prior Art
A hybrid electric vehicle (HEV) is a vehicle configured with a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. As one non-limiting example, a hybrid propulsion system may combine a conventional propulsion system that includes an internal combustion engine and a stepped-ratio automatic transmission with an electric propulsion system that includes one or more electric motors and a rechargeable energy storage device, such as a battery, that can power the electric motors or store energy to improve fuel economy over the conventional vehicle. A hybrid electric vehicle typically provides different powertrain operating modes with the engine running or shutdown depending on the vehicle operating conditions, battery conditions, and driver's propulsion request. Hence, one of the major functions that an HEV provides is the ability to start or stop the engine during certain conditions. When the engine is running, the electric portion of the propulsion system may be used to assist the engine in providing the required vehicle propulsion. During the conditions when the engine is shutdown, the driver's propulsion request can be provided entirely by the electric motor.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology to reduce fuel consumption, but without use of a hybrid drivetrain. In such vehicles, called “micro-HEVs”, shutting down the engine during conditions where the engine operates at idle speed will be used to reduce fuel consumption and reduce emissions in a conventional powertrain that includes an internal combustion engine and a stepped-ratio automatic transmission, but no electric machine for driving the wheels. The primary condition that is checked by the micro-HEV powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine.
In a micro-HEV application using an internal combustion engine with an enhanced starter motor for engine start/stop and a dual-clutch (DCT) automatic transmission, as shown in FIGS. 1 and 2, it is important to provide vehicle propulsion upon an engine restart in a responsive, consistent, and predictable manner. Delays due to transmission engagement and clutch torque capacity application will directly result in poor vehicle propulsion response.
Premature clutch torque capacity application can also lead to driveline torque oscillations and potential engine stall while restarting. In addition, poor vehicle performance will be sensed by the driver if the transmission engagement feel is too harsh during or after the engine restart. Temperature and other environmental conditions may further amplify these issues.
A powertrain control system for a micro-HEV powertrain must also provide an immediate response to a request for vehicle propulsion when the engine is restarted after the gear shift lever was moved from the Drive position to the Reverse position, or from Reverse to Drive during the time while the engine was off.
For example, during an engine stop event while the gear shifter is in Drive and the brake pedal is depressed, the driver can move the shifter into Reverse while continuing to depress the brake pedal, thereby keeping the engine off while the vehicle is stopped. Given this condition, if the transmission is not engaged into reverse gear while the engine is off and the vehicle is stopped at the time the driver moves the gear shifter to Reverse position, there will be an excessive delay in vehicle propulsion once the driver releases the brake and the engine is started. The excessive vehicle propulsion delay is due to delaying the transmission engagement into reverse gear until the engine is started. In DCT applications in which the first and reverse gears are applied on the same shaft (e.g. odd shaft), if the transmission is engaged into reverse at the time of the engine start rather than at the time that the gear shifter was selected to Reverse, the gearbox shift synchronizers within the DCT must be disengaged from first gear, engaged into reverse gear, and then the clutch must be filled and stroked before any clutch torque can be applied. In DCT applications in which the first and reverse gears are applied on different shafts (e.g. 1st gear on odd, reverse on even), the gearbox shift synchronizer must be engaged into reverse gear and then the clutch must be filled and stroked before applying any clutch torque.
Since vehicle propulsion cannot be provided until the clutch torque is applied, the delays in gearbox engagement and clutch actuation directly result in wheel torque delays and poor vehicle propulsion response.
A powertrain control strategy is needed to avoid this problem in a micro-HEV with a dual-clutch transmission.